An organic electroluminescent element (may hereinafter be referred to as an organic EL element) is a self light-emitting element, and is thus brighter, better in visibility, and capable of clearer display than a liquid crystal element. Hence, active researches have been conducted on organic EL elements.
In 1987, C. W. Tang et al. of Eastman Kodak developed a laminated structure element sharing various roles among different materials, thereby imparting practical applicability to organic EL elements using organic materials. They laminated a layer of a fluorescent substance capable of transporting electrons, namely, tris(8-hydroxyquinoline)aluminum (will hereinafter be abbreviated as Alq3), and a layer of an aromatic amine compound capable of transporting holes, and injecting the charges of electrons and holes into the layer of the fluorescent substance to perform light emission, thereby obtaining a high luminance of 1,000 cd/m2 or more at a voltage of 10V or less (see Patent Document 1 and Patent Document 2).
Many improvements have been made to date for commercialization of organic EL elements. For example, there is known an electroluminescent element sharing the various roles among more types of materials, and having a positive electrode, a hole injection layer, a hole transport layer, a light emission layer, an electron transport layer, an electron injection layer, and a negative electrode provided in sequence on a substrate. High efficiency and durability are achieved by such an element.
For a further increase in the luminous efficiency, it has been attempted to utilize triplet excitons, and the utilization of phosphorescent light emitting substances has been considered.
Furthermore, elements utilizing light emission by thermally activated delayed fluorescence (TADF) have been developed, and an external quantum efficiency of 5.3% has been realized by an element using a thermally activated delayed fluorescence material.
The light emission layer can also be prepared by doping a charge transporting compound, generally called a host material, with a fluorescent substance or a phosphorescent light emitting substance. The selection of an organic material in the organic EL element greatly affects the characteristics of the element, such as efficiency and durability.
With the organic EL element, charges injected from both electrodes recombine in the light emission layer to obtain light emission, and how efficiently the charges of the holes and electrons are passed on to the light emission layer is of importance. For example, hole injecting properties are enhanced, and the properties of blocking electrons injected from the negative electrode are enhanced to increase the probability of holes and electrons recombining, and excitons generated within the light emission layer are confined, whereby a high luminous efficiency can be obtained. Thus, the role of the hole transport material is so important that there has been a desire for a hole transport material having high hole injection properties, allowing marked hole mobility, possessing high electron blocking properties, and having high durability to electrons.
In connection with the life of the element, heat resistance and amorphism of the material are also important. A material with low thermal resistance is thermally decomposed even at a low temperature by heat produced during element driving, and the material deteriorates. In a material with low amorphism, crystallization of a thin film occurs even in a short time, and the element deteriorates. Thus, high resistance to heat and satisfactory amorphism are required of the material to be used.
As the hole transport materials hitherto used for organic EL elements, N,N′-diphenyl-N,N′-di(α-naphthyl)benzidine (hereinafter NPD, for short) and various aromatic amine derivatives are known (see Patent Document 1 and Patent Document 2). NPD has a satisfactory ability to transport holes, but its glass transition point (Tg) as an index to heat resistance is as low as 96° C., and deterioration of the element characteristics due to crystallization occurs under high temperature conditions. Among the aromatic amine derivatives described in Patent Document 1 and Patent Document 2 are compounds having an excellent hole mobility as high as 10−3 cm2/Vs or more. However, their electron blocking properties are insufficient, thus posing the problem that some of the electrons pass through the light emission layer, and an improvement in the luminous efficiency cannot be expected. Thus, a material having high electron blocking properties, providing a more stable thin film, and higher resistance to heat has been desired for an even higher efficiency.
As compounds increased in efficiency and improved in characteristics such as hole transport properties, proposals have been made for arylamine compounds having a substituted indenoindole structure (Compounds A to B) expressed by the following formulas (see Patent Documents 3 to 4):

However, Compound A has only been used as a host material, and an element using Compound B for a hole transport layer has been improved in luminous efficiency, but the improvement has been still insufficient. Thus, there has been a desire for the development of a material which achieves an even lower driving voltage and an even higher luminous efficiency while enhancing heat resistance.